The present invention relates to a fixing apparatus in an image forming apparatus.
An image forming apparatus employing electrophotography is provided with a fixing apparatus for fixing a toner image formed on a printout material. Some types of fixing apparatuses use a heating belt. Such a fixing apparatus comprises a heating roller, a belt support roller, a heating belt, and a press roller. The belt support roller is made of an elastic material. The heating belt is supported by the heating roller and the belt support roller, and is rotated by rotations of both rollers. The heating roller heats the heating belt. The press roller is rotated by pressing against and contact with a portion supported by the belt support roller in the heating belt. Recording paper is fed by a contact area between the heating belt and the press roller. A toner image is pressurized and heated, and is fixed onto the recording paper.
The belt support roller is deformed due to elasticity along a peripheral direction of the press roller and can allow the heating belt having a large width to contact the press roller. Accordingly, it is possible to enlarge the width of the contact area between the heating belt and the press roller, and to fix toner images by transporting the recording paper sandwiched between wide areas.
However, the conventional fixing apparatus always contacts the heating belt, i.e., the belt support roller under a constant pressure. When there is a wide contact area between the heating belt and the press roller, a contact pressure is decreased at the contact area per unit area. When the recording paper is thick, the contact area between the heating belt and the press roller cannot generate a high enough pressure to press the recording paper, then that pressure gives an unfavorable effect on fixing. The width of the contact area between the heating belt and the press roller may vary with aging of materials. When the contact area width varies, a fixing condition of toner images are changed by varying the contact pressure per unit area on the contact area.
Accordingly, it is requested to change the width and the contact pressure of the contact area between the heating belt and the press roller according to various conditions.
Conventionally, there are offered proposals for solving this problem. One proposal is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-72931. The fixing apparatus uses a roller attached to a rotative lever to support the heating belt. A solenoid rotates the rotative lever to displace the roller. The heating belt changes the condition of contact with the press roller.
When a change occurs in the condition of contact between the heating belt and the press roller on the fixing apparatus, however, the height position of the contact area varies in a vertical direction. As the conditions of conveying recording paper are changed the conveying may stop or the fixing conditions of toner images may be changed.
Another proposal is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-115003. In the fixing apparatus, recording paper is placed on a guide plate. A cylindrical heating belt rotates in contact with the recording paper. As the guide plate is displaced by a pressing means, the condition of contact between the heating belt and the guide plate is changed.
When a change occurs in the condition of contact between the heating belt and the guide plate on the fixing apparatus, however, the height position of the contact area varies in a vertical direction. Since conditions of conveying for recording paper is changed the conveyance may stop and fixing condition of toner images may be changed.
It is an object of the present invention to provide a fixing apparatus for solving the conventional problems.
A fixing apparatus according to an aspect of the present invention comprises: a heating member which rotates around an axis; a belt support member which is heat-resistant and elastic, has a cylindrical shape partly cut out along an axis line direction, and is capable of varying the size of a cross section area; an endless heating belt which rotates by being supported by the heating member and the belt support member, and slidingly contacts the belt support member; a press member which conveys the printout material by cooperating with part of the heating belt supported by the belt support member putting the printout material between them; a first control section which supports the belt support member and controls the size of the cross section area thereof; and a second control section which applies a force to the heating belt toward the press member and controls that magnitude.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.